Melt butter in a heavy 6-quart pot over medium-high heat. Add half the lamb; cook, stirring 2 or 3 times, 4 minutes or until meat loses its pink color. Remove to bowl with slotted spoon. Repeat with remaining lamb. Pour off all but 1/2 cup drippings. Add onions and garlic to pot and cook about 4 minutes, stirring once, until lightly browned. Stir in curry powder and salt. Reduce heat to low and cook 3 minutes to develop flavor. Add tomatoes, tomato paste, water and the lamb along with any accululated juices. Cover and bring to a simmer over medium heat. Simmer 40 minutes, stirring 2 or 3 times. Stir in remaining ingredients; simmer covered 20 minutes more or until meat is tender.
